In 1973 Tardif signed with the World Hockey Association, playing with the Los Angeles Sharks. He was the Sharks' leading scorer that season, and was named to play for Team Canada in the 1974 Summit Series the following fall. The Sharks, however, finished with the league's poorest record, and moved to Detroit, where Tardif played brilliantly before a trade to the Quebec Nordiques.

In Quebec, Tardif became one of the league's preeminent stars. He finished the 1975 season with 50 goals, and added a league-leading ten goals in the playoffs en route to the AVCO Cup finals against the eventual champion Houston Aeros. The next season he led the WHA in goals, assists and points by wide margins and becoming only the second professional player to score seventy goals in a single season, while the Nordiques rampaged to fifty wins. Tardif's playoff was cut short after an attack by Calgary Cowboys goon Rick Jodzio in which he incurred serious head injuries, leading to one of the first ever cases where a hockey player was charged in a court of law for assault.

The next season Tardif was named the captain of the team, and recovered to post another hundred-point campaign while leading the Nordiques to its only WHA championship, and followed that up in 1978 with a 154-point campaign (setting a professional hockey record eventually broken by Wayne Gretzky), for which he received his second league MVP award.

He remained a star when the Nordiques joined the NHL after the WHA folded in 1979, acting as the team's first NHL captain. Tardif retired after the 1983 season, and the Nordiques retired his number #8 jersey in tribute to their first great scoring star. He finished his career scoring 316 goals and 350 assists for 666 points in the WHA, and 194 goals and 207 assists for 401 points in the NHL. He currently owns a car dealership in Quebec City.